It is known that ultraviolet rays cause various changes in the skin. In the field of skin science, ultraviolet rays are divided into long wavelength ultraviolet ray (400-320 nm), medium wavelength ultraviolet ray (320-290 nm) and short wavelength ultraviolet ray (290 nm or shorter), which are called UV-A, UV-B and UV-C, respectively. Ultraviolet rays in the sunlight consist of UV-A and UV-B, since UV-C hardly reaches the earth due to absorption in the ozone layers.
When the skin is irradiated with a certain quantity of UV-B, various changes occur in the skin such as formation of erythema or bulla and acceleration of melanism which subsequently causes pigmentation. Destruction of the ozone layers in the sky caused by the recently increasing air pollution led to a grave social problem, because such a destruction increases quantity of UV-B in the sunlight. On the other hand, irradiation with UV-A not only leads to immediate onset of melanism in the skin (immediate melanism) but also changes conditions of elastic fibers in blood vessel walls and connective tissues because its energy penetrates epidermis and reaches dermal side of the skin. It is considered in general that such functions of both UV-A and UV-B accelerate aging of the skin, cause generation of spots, wrinkles, freckles and the like and, in a long-term manner, cause skin carcinoma.
With revelation of the influence of ultraviolet rays on the human skin, attempts have been made to develop a compound which can absorb UV-A and UV-B. It is desirable that such an ultraviolet ray absorbent can satisfy all necessary requirements which include: (1) complete absorption of UV-A or UV-B light as far as is possible; (2) stable to light and heat; (3) no toxicity or stimulus to the skin or other hazardous effects; (4) prolonged action; and (5) excellent compatibility with a cosmetic base.
Dibenzoylmethane derivatives have been used as UV-A absorbents, and derivatives of cinnamic acid esters, benzophenone, p-aminobenzoic acid, salicylic acid and the like have been used as UV-B absorbents.
These prior art ultraviolet ray absorbents, however, do not fully satisfy the aforementioned requirements. Especially, they have inferior stability to light and therefore are decomposed or become reactive when exposed to ultraviolet rays as described in Int. J. Cosmetic Science, Vol.10, p.53 (1988). Such a decomposition spoils prolonged action of the prior art ultraviolet ray absorbents and also the decomposed products themselves or their reaction products with compounding ingredients exert significant influence on the skin as described in Fragrance Journal, Vol.84, p.34 (1987).
In consequence, great concern has been directed toward the development of an ultraviolet ray absorbent which can satisfy all of the aforementioned requirements, especially excellent stability to light.